Remove the herbs stalks, onion and garlic from the roasting tray - reserve the garlic. Put the tray over a gas hob and pour in 100 millilitres of water. De-glaze the pan over a medium heat, stirring with a spatula and scraping up any bits stuck to the tray. When you have just a few tablespoons of liquid left, add in the flour and cook for a minute or so until blended. Gradually whisk in the stock then simmer for 5 minutes, stirring constantly, until thickened. Squeeze the cooked garlic from the cloves, add back to the pan, and mix in well. Once cooked, pour the gravy into a large jug and cover the top with cling film to keep the heat in and stop a skin forming.